[maui-labs docs] Document Linux GTK4 Platform Backend and CSS Styling Updates #3537

## Linux / GTK4

The Linux GTK4 platform backend brings .NET MAUI applications to Linux desktops using GTK4 rendering via GirCore bindings.

### Supported Controls

- **Basic Controls**: Label, Button, Entry, Editor, CheckBox, Switch, Slider, Stepper, ProgressBar, ActivityIndicator, Image, ImageButton, BoxView, RadioButton
- **Input & Selection**: Picker, DatePicker, TimePicker, SearchBar
- **Collections**: CollectionView (virtualized), ListView, TableView, CarouselView, SwipeView, RefreshView, IndicatorView
- **Layouts**: StackLayout, Grid, FlexLayout, AbsoluteLayout, ScrollView, ContentView, Border, Frame
- **Pages & Navigation**: ContentPage, NavigationPage, TabbedPage, FlyoutPage, Shell
- **Shapes**: Rectangle, Ellipse, Line, Path, Polygon, Polyline (Cairo-rendered)
- **Other**: GraphicsView (Cairo), WebView (WebKitGTK)

### Platform Features

- Native GTK4 rendering with every control mapped to a real GTK4 widget
- Blazor Hybrid support for hosting Blazor components in GTK windows
- Full gesture support: Tap, Pan, Swipe, Pinch, and Pointer gestures
- Animations: TranslateTo, FadeTo, ScaleTo, RotateTo at ~60fps
- Transforms: TranslationX/Y, Rotation, Scale via Gsk.Transform
- VisualStateManager with Normal, PointerOver, Pressed, Disabled, Focused states
- ControlTemplate support with ContentPresenter
- Font icon support with embedded font registration
- FormattedText with Pango markup (colors, fonts, sizes, formatting)
- Alerts, dialogs, and modal pages via GTK4 windows
- Brushes and gradients (SolidColor, LinearGradient, RadialGradient)
- MenuBar and context menus
- Automatic light/dark theme detection

### CSS Styling

The GTK4 backend uses CSS for styling. Font sizes are specified in logical pixels (px), matching MAUI's device-independent units. Handler CSS is composed per widget and preserved across updates:

- Font, color, spacing, background, and border updates preserve one another
- Custom handlers should pass complete CSS fragments on every update
- Use `property` key for distinct independently-updated properties
- Most recently updated fragment wins for overlapping declarations

### Quick Start

Install the GTK4 platform:

\`\`\`bash
dotnet add package Microsoft.Maui.Platforms.Linux.Gtk4 --prerelease
\`\`\`

Add GTK4 to your MauiProgram:

\`\`\`csharp
var builder = MauiApp.CreateBuilder()
    .UseMauiApp(App)()
    .UseGtk4();  // Enable GTK4 platform

var app = builder.Build();
app.Run();
\`\`\`

For more details, see [Linux GTK4 Platform README](https://github.com/dotnet/maui-labs/blob/main/platforms/Linux.Gtk4/README.md) in the maui-labs repository.